Traveling wave electron discharge devices



July 1957 J. H. BRYANT ET AL 2,800,603

TRAVELING WAVE ELECTRON DISCHARGE DEVICES Filed Aug. 23, 1952 2Sheets-Sheet l INVENTORS JOHN H. BRYANT ALBERT r1. PE/FER ROBERT wW/LMARTH TRAVELING WAVE ELECTRON DISCHARGE DEVICES Filed Aug. 25, 1952 2Sheets-Sheet 2 INVENTORS JOHN H. BRYANT ALBERT r1- PE/FER ROBERT ww/LMARTH AZ K445 A ORNEY United Sttes TRAVELING WAVE ELECTRON DISCHARGEDEVICES John H. Bryant and Albert G. Peifer, Nutley, and Robert W.Wilmarth, Rutherford, N. J., assignors to International Telephone andTelegraph Corporation, a corporation of Maryland Application August 23,1952, Serial No. 306,012

11 Claims. (Cl. SIS-3.5)

This invention relates to traveling wave electron discharge devices andmore particularly to the radio frequency propagating structure, theelectron gun and coupling arrangements therefor.

The traveling wave type of tube is particularly useful in widebandmicrowave systems since it is capable of amplifying radio frequencyenergy over an unusually wideband of frequencies. The tube includes aform of propagating structure, usually a helix, for transmission ofmicrowave energy for interaction with an electron beam closelyassociated with the structure. The helical characteristic of thepropagating structure is such that the helix velocity of microwavesignals conducted along the helical path is approximately the same as orslightly slower than the velocity of the electrons of the beam, wherebythe electrical field of the microwave signals interacts with theelectron beam for amplification of the microwave signals.

In applications of such structure it is desirable that noise in thetraveling wave tube be as low as possible, such as where employed inreceiving equipment for detection of weak signals. The source of noisein a traveling wave tube is the electron beam. At least part of thisnoise is carried in the space charge by plasma waves which arecharacterized by alternate maximum of electron velocity and currentfluctuations. These waves originate from random emission of electronsfrom the cathode. One means of achieving a low noise figure may beobtained by beginning the helix, or other wave interaction structure, ata judiciously chosen position along the beam, and that is at the pointat which the noise in the beam will couple least effectively to thehelix. This position is determined by an optimum relation betweenvelocity and current modulation magnitude and occurs near a currentmodulation maximum. This position may be theo retically derived frompresently available theory by one skilled in the art.

A further improvement in the noise figure can be obtained by employingin the traveling wave tube a potential jump electron gun structure. Withsuch an arrangement, the accelerating potential is applied to the beamin two or more stages. In a two stage arrangement the electrons areaccelerated in the cathode-anode region to the first anode potential andthen allowed to drift a specified distance at the first anode potential.In a spacecharge-limited diode the magnitude of the velocityfluctuations for practical purposes, is independent of the anodepotential. Thus, the velocity fluctuations in the first drift tube atlow potential are the same as would have been obtained if the same beamcurrent had been achieved at full helix potential. By jumping to thefull helix potential at the proper position in this first drift tube,however, a reduction in the magnitude of the velocity fluctuations isachieved. This jump is made to occur at a velocity fluctuation maximum.From conservation of energy considerations the velocity fluctuations arereduced inversely as the square root of the voltage ratio. On theother'hand, the noise figure varies as :the squareof the ice.

velocity fluctuations, so that the noise figure varies inversely as theratio of the voltages. For example, a voltage jump of volts to 600 voltsshould give a noise figure reduction of 4 to 1 in power, or 6 db. Inpractice it is found to give a 3 to 4 db improvement in noise figure.Reasons may be found for this reduction in noise figure improvement.They include finite distance over which the beam voltage changes, finitediameter of the electron beam, and thermal velocity fluctuations in theelectron beam.

To assure that the forward end of the propagating structure ismaintained at the same potential as the potential on that portion of thelast drift tube exending from the last anode it is necessary to apply aD. C. potential to the propagating structure and the elementincorporated in the forward end of the helical structure forming theremaining portion of the last drift tube. To accomplish the required D.C. connection thereto it is necessary that the D. C. source be isolatedfrom the radio frequency energy being coupled to thepropagatingstructure, thus assuring that the propagating structure remains anintegral part of the radio frequency section of a traveling wave.

It is an object of the present invention to provide a. traveling wavetube having an electron gun and propagating structure arrangement whichproduces an improvement noise figure as hereinabove outlined.

Another object of this invention is to provide a potential jump electrongun to aid in the improvement of the noise figure.

Another object of this invention is the judicious positioning of thepropagating structure of a traveling wave tube with respect to theeletcron emission surface to aid in the reduction of noise.

A feature of this invention is to provide an element in the inputmatching section of the propagating structure as a portion of the driftspace aiding in the reduction of noise.

Another feature of this invention is to provide the element in the inputmatching section employed as a portion of the drift space with a D. C.potential to reduce the possibility of a static charge building up onthe forward end of the propagating structure to block the electron beam.

Another feature of this invention is to provide two or more stages inthe electron gun, each stage including a drift space, for applyingaccelerating potential to the beam.

A further feature of this invention is to provide a propagatingstructure of a traveling wave tube with D. C. isolation and yetremaining a portion of the R.-F. circuit.

The above mentioned and other features and objects of this inventionwill become more apparent by reference to the following descriptiontaken in conjunction with the accompanying drawings, in which:

Figs. 1A and 1B are partial longitudinal sectional views of anembodiment of a traveling Wave tube illustrating an electron gun and theforward portion of the radio frequency propagating structure thereof inaccordance with the principles of our invention;

Fig. 2 is a cross-sectional view taken along line 2-2 of Fig. 1A;

Fig. 3 is a cross-sectional view taken along line 3-3 of Fig. 1B; and

Fig. 4 is a cross-sectional view taken along line 4-4 of Fig. 1B.

Figs. 1A and 1B,'when placed end to end, illustrate a partiallongitudinal sectional view of an embodiment of a traveling wave tubecomprising a metallic housing 1, a potential jump electron gun 2, and aradio frequency section 3 including a wave propagating structure 4,shown herein to be a helical conductor 5 each turn of which is bonded tothree dielectric supportingmembers fiend.

41 The desired. magnetic fi'eldmay be. produced by anelect'romagnetassembly or a permanent magnet assenra bly as may be desiredl Referringmore specifically to Figs. IA and 2; potential jumpelectron gun 2 isshown to comprise a plurality of electrodes 9- -12 and support members13' and 14, each having a triangular shaped periphery whose central'area is circular and. disposed coaxially of the electron beam path.. Theelectrodes. 9 -12 and support members 13' and'14 are securedlinpositionand alignmentirelative. to each other by meansofbondingto. dielectricrods 15, oneeach situated at each corner of the triangular. area. Toproperly align electron gun 2'with respect to the housing 1 andthepropagating structure 4, support member 14 is brazed to the non-magneticbarrier 16 disposed crosswise of'housing lin the proper positionrelative to said housing. Barrier 16, mechanically carrying electron gun2, is secured in proper alignment by means of bonding to metallic rods17extending from magnetic barrier 18 located'near the forward end of thehousing 1.

Electrode 9' comprises a cylinder 19 extending therefrom to enclose thefilament winding 20 and carrying on the end thereof an oxide coatedsurface 21' for emitting the. electrons forming the electron beam.Electrode 10 coaxial of cylinder 19 is the intensity and focusingelectrode of the. gun assembly. Electrode 11, the first anode of themulti-anode gun assembly 2, has connected thereto electrically andmechanically the first drift tube 22. Electrde'12, the second anode hasconnected thereto the forward portion of the second drift tube 23 whichis electrically' and mechanically connected to support member 13whoseprimary purpose is mechanical support for said forward-portion of thesecond drift tube. The support member 14 not only provides mechanicalsupport and alignment for electrodes 9-12 and support member 13 but alsoserves to conduct heat to the plate 16.

Since the electron gun assembly 2 is so arranged to provide-animprovement in the noise figure, the drift tubes have a predeterminedlength to cooperate with the electrode voltages such that each potentialjump occurs at a predetermined time corresponding to a velocityfluctuation maximum as set forth hereinabove. Representative voltagesthat may be applied'to the electrodes where the drift tubes haverelative lengths substantially as shown, are 600 volts for electrode 9,350 volts for electrode 11 and drift tube 22, and ground potential forelectrode 12 and the forward portion 23 of the second drift tube,therearward portion of which includes an element of the coupling section7 extending to the first turn of conductor 5. The intensity electrodehas applied thereto a variable negative potential, depending upon theapplication of the device, with respect to the electrode 9.

Referring to Figs. 18, 3, and 4, the mechanical and electricalrelationship between coupling section 7 and the rearward end of theelectron gun 2 is illustrated. The forward end of the radio frequencysection3 carries a triangular shaped element 24 secured to a cylinder25. To provide proper axial alignment of section 3 with the projectedelectron beam, element 24 is slidably received in cylinder 26 whichabuts barrier 16 and is brazed thereto in the proper position relativeto thepath of the electron beam. The slidable association betweencylinder 26 and element 24permits differential rcoefficients. ofexpansion between electron gun 2 and'R.-F. section 3.

Element 24 includes an aperture 27 and carries a eucular plate 28-thereon'having an aperture therein in alignment with aperture 27; Theaperture in plate 28 has an offset portion 28a therein through which theD. C. potentiallead 29 is' disposedto apply the required-D. C. potentialto the R.-F. coupling section 7 and hence to the conductor 5} the firstturn of'which is judiciously positioned to aid in improvement of thenoise figure.

Element 24 further provides a recession 30 whereby the coupling section7 may be secured and axially positioned. Coupling section..7*incli1desan outer conductive cylinder 31.having therein an. aperture forthe.receipt of the inner conductor 32'of coaxial Rl-F. input waveguide 33.Immediately within cylinder 31 is a dielectric sleeve 34'whicl1' may beformed, for example, by a. glass braid' or ceramic glazing materialannealed at elevated temperatures. Surroundedbysleeve 34 and encirclingdielectric support rods 6 is an inner conductive cylinder 35 to whichthe required D. C. potential is applied by lead 29 and to which iselectrically connected conductor 5. Within the space provided by thearrangement of rods 6 and having a diameter approximately equal to thatof conductor 5.is a metallic sleeve 36, electrically connected tocylinder 36. by strap .37. The coaxial arrangement of section 7 not onlyprovides the requiredD. C. isolation of conductor 5, still coupled froman R.-F. view point to waveguide 33, but providesa means to mechanicallysupport conductor 5 coaxially of the path of the electron beam.

Conductive cylinder 35, considered as an'electrode of the device, ismaintained at-a D. C. potential corresponding to the-potential on thefOIWflId'POI'tlOI'l 23 of the second drift tube and at the sametimeforms a'portion of the rearwardsectionof the second drift tube whichextends tothe first turn'of the helix. The radio frequency energy isconducted along inner conductor 32 to cylinder 31, then capacitivelycoupled to cylinder 35 through insulating sleeve 34, and hence, toconductor 5. Conductivesleeve 36-is maintained at ground potential toprevent a staticcharge-from'buildingupbetween sleeve 36 and cylinder 35wherein rods 6 are of dielectric materials. If this charge did build-upit would block the electron beam. The dielectric' sleeve 34 thus allowsthe connection of a D. C. potential to the conductor 5 while yetproviding the coupling of R.-F. energy to the conductor 5.

While we have described above the principles of our invention inconnection with specific apparatus, it is to be clearly understood thatthis description is made only by way of example and not as a limitationto the scope of our invention as set forth in the objects thereof and inthe accompanying claims.

We claim:

1. In a traveling wave electron discharge device, an electron beam guncomprising a cathode for emitting electrons, a plurality of electrodesto confine said electrons into an electron beam, one of said electrodesincluding a first anode surrounding the path of said beam and carryingtherewith a first drift tube disposed coaxially of said beam path,another of said electrodes including a second anode disposed closelyadjacent the output of said first drift tube and carrying therewith asecond drift tube also disposed coaxially of said beam path, means toapply a given directcurrent potential to said anode, means to apply adirect current potential higher than said given potential to said secondanode, means to support said cathode and said electrodes as an electrodeunit including an annular conductive member disposed in spaced relationabout said second drift tube and means supporting said electrode unitfrom said member, a conductive cylinder carried by said annular membercoaxially of said beam path, a radio frequency propagating structuredisposed parallel to said beam path and means carried by said cylinderto support one end of said radio frequency propagating structure foraxial movement relative to said annular member.

2. In a traveling wave electron discharge device according to claim 1,wherein the means carried by said cylinder comprises: aconductive platehavingperipherally spaced portions slidably engaging the inner surfaceof said cylinder.

3. In a traveling wave electron discharge device according to claim 2,wherein said radio frequency propagating structure includes a radiofrequency conductor and a radio frequency transition section, saidtransition section comprising a pair of concentric conductive cylindersspaced apart by a layer of dielectric material, the outer cylinder ofsaid pair being carried by said plate and the inner cylinder of saidpair being coupled to said radio frequency conductor.

4. In a traveling wave electron discharge device according to claim 3,wherein the inner cylinder of said pair is electrically isolated withrespect to said plate and means are provided to apply potential to saidinner cylinder.

5. in a traveling wave electron discharge device having an electrodeunit for producing a beam of electrons and a radio frequency propagatingstructure disposed parallel to the axis of the path of said beam; aconductive housing for said electrode unit, a conductive annularsupporting member carried by said housing concentrically of said beampath, means to support said electrode unit on one side of said member, aconductive cylinder carried on the other side of said member coaxiallyof said beam path and means carried by said cylinder to support one endof said radio frequency propagating structure in parallel relation tosaid beam path.

6. In a traveling wave electron discharge device according to claim 5,wherein the means carried by said cylinder comprises a conductive plateperipheral portions of which slidably engage the inner surface of saidcylinder.

7. In a traveling wave electron discharge device accord ing to claim 6,wherein said radio frequency propagating structure includes a radiofrequency conductor and a radio frequency transition section, saidtransition section comprising a pair of concentric conductive cylindersspaced apart by a layer of dielectric material, the outer cylinder ofsaid pair being carried by said plate and the inner cylinder of saidpair being connected to said radio frequency conductor.

8. In a traveling Wave electron discharge device having an electrodeunit for producing a beam of electrons and a radio frequency propagatingstructure disposed parallel to the axis of the path of said beam; saidstructure comprising a helical conductor, a radio frequency transitionunit for said helical conductor, said transition unit comprising threeconcentrically disposed conductive cylinders, a layer of dielectricdisposed between the inner and middle cylinders to provide capacitivecoupling therebetween, the inner cylinder being connected to saidhelical conductor, means coupling radio frequency energy between themiddle and outer cylinders forpropagation along said helical conductor,and means to apply a given D. C. potential to said inner cylinder.

9. in a traveling wave electron discharge device according to claim 8,further including a conductive cylindrical support, a conductive plateperipheral portions of which are receivable in sliding engagement withthe inner surface of said cylindrical support and means supporting oneend of said radio frequency propagating structure on said plate.

10. in a traveling wave electron discharge device according to claim 9,wherein the means for supporting said radio frequency structure on saidplate includes a plurality of ceramic rods disposed to support saidhelical conductor, a pair of concentrically spaced conductive cylinders,the inner of said cylinders being disposed about said rods and coupledto said helical conductor, and the outer of said cylinders beingconnected to a radio frequency transmission line.

11. In a traveling wave electron discharge device ac cording to claim10, wherein the means for supporting one end of the radio frequencystructure on said plate includes an abutment to engage the ends of saidrods.

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